A phase detector is a circuit or instrument for detecting a difference in phase between corresponding points on two signals. Ordinary phase detectors of radio frequency signals at 2 GHz use a mixer as a phase detector. FIG. 1 illustrates such a phase detector 10 comprising amplitude limiters 12, 14 and a mixer 16. Signals S.sub.a and S.sub.b are provided as inputs to amplitude limiters 12, 14, respectively. For purposes of illustration, signal S.sub.a is a 1.999 GHz signal and S.sub.b is a 2.000 GHz signal. Amplitude limiters 12, 14 saturate the amplitudes of signals S.sub.a and S.sub.b to produce output signals S.sub.12 and S.sub.14, thereby eliminating amplitude variations between signals S.sub.a and S.sub.b. Note that signals S.sub.12 and S.sub.14 are signals representative of signals S.sub.a and S.sub.b, respectively. Signals S.sub.12 and S.sub.14 are mixed by mixer 16 to produce output signal S.sub.16. Since signals S.sub.12 and S.sub.14 have 360.degree. of phase shift between them (i.e., S.sub.a is 1.999 GHz and S.sub.b is 2.000 GHz), mixer 16 will produce a sine wave output signal (i.e., signal S.sub.16 is a sine wave output signal). See FIG. 2, which depicts a sine wave output signal 20.
Using the phase detector 10 of FIG. 1 (and other similar phase detectors) in a feedback system to control the phase of one or more signals has certain limitations. Particularly, the feedback system would be limited to detecting phase differences of .+-.90.degree. because of direction reversal by signal S.sub.16. For example, if signals S.sub.a and S.sub.b have a phase difference of +45.degree. (indicated by point A), a voltage value of 0.5 is indicated. The +45.degree. phase difference is indicated by point A on FIG. 2. At 90.degree., the direction of signal S.sub.16 reverses and at a +135.degree. phase difference between signals S.sub.a and S.sub.b (indicated by point B), a voltage value of 0.5 is also indicated. Thus, the prior art phase detector 10 uses the same voltage value to indicate different phase differences. This, in effect, limits the range of phase detector 10 (and the feedback system) to .+-.90.degree..
In some instances, a feedback system incorporating the phase detector 10 of FIG. 1 (and other similar phase detectors) is sufficient for detecting phase differences between two signals if the phase differences should be within .+-.90.degree.. However, in instances where the phase differences between two signals to be phase detected will be beyond .+-.90.degree., e.g., 135.degree., such phase detector 10 (or equivalents) will be inadequate since a same voltage value may indicate multiple phase differences. The ideal phase detector should be monotonic for measuring phase differences within a range of .+-.180.degree.. That is, the ideal phase detector should indicate an absolute voltage value for each phase difference between .+-.180.degree.. Accordingly, there exists a need for such a phase detector.